Compression limiting circuits are widely used to prevent overmodulation in AM and FM transmitters, to prevent overload distortion in audio amplifiers, and to prevent overload distortion in hearing aids and/or to prevent loudness discomfort for the wearer of the hearing aid. In operation, such circuits automatically reduce the overall system gain once the input signal level exceeds a certain threshold value. Such circuits are typically designed with a compression ratio of 5:1 or greater (each 5 dB increase in input above the threshold value results in only 1 dB or less increase in output) and with a rapid reduction in gain with a sudden increase in input level and a relatively slower recovery to full gain after the input level has dropped below the threshold value. The response time of such circuits is commonly characterized by two numbers; the "attack time" defined as the time required for system gain to stabilize to within 1 or 2 dB of its asymptotic value after a sudden increase in input level, and the "recovery time" defined as the time required for the system gain to recover to within 1 or 2 dB of full gain after the input signal level has dropped below the threshold value.
A common problem with such circuits is that no one choice of recovery time constant appears adequate for all situations: A long recovery time will produce audible gaps or "holes" in the program material if that program material contains short intense transients; a recovery time short enough to prevent such holes will cause audible "breathing" and a "noisy" sound such as is commonly heard in live baseball broadcasts.
A solution to these problems has been to use a variable-recovery-time circuit such as described in a paper of G. A. Singer, in Audio Engineering, November 1950, pp. 18-19 and 69-70 entitled "New Limiting Amplifier." The application of variable-recovery-time circuits to compression-limiting circuits in hearing aids was disclosed by David Hotvet in U.S. Pat. No. 4,718,099, issued Jan. 5, 1988 and assigned to Telex Communications, Inc. The Hotvet circuit has been successfully used under the commercial name of "adaptive compression" by Telex Communications, substantially improving the operation of their compression-limiting hearing aid circuits. The improvement can be heard on the recorded demonstration cassette tape available from Telex corporation under the title "Telex Adaptive Compression Fitter Demonstration, #HA-110" originally issued in 1988.
In contrast to output-limiting type circuits, which act to prevent the output signal levels from exceeding a certain value, the principal applicant (Killion) has developed a wide-dynamic-range logarithmic-compression (WDRLC) circuit which does nothing to limit signal levels above its threshold of operation, but instead acts to increase system gain for weaker (below-threshold) signals. This WDRLC circuit was disclosed in Killion U.S. Pat. No. 4,170,720, issued Oct. 9, 1979, hereafter called the -720 patent. In further contrast to output-limiting compression circuits, the operation of the WDRLC circuit permits the necessary change in gain with signal level to be spread out over a 50 to 60 dB range of input levels, with corresponding smaller compression ratios; a value of 2:1 is common, with compression ratios of 1.2:1 up to a maximum of 3:1 sometimes used.
The contrast between limiting-type and WDRLC-type circuits is easily seen in hearing aid applications, where the limiting-type circuit acts to limit the maximum hearing aid output for loud sounds and does nothing to affect the gain for quiet sounds, whereas the WDRLC circuit acts to increase the gain of the hearing aid for quiet sounds and provides limitless "transparent" operation for loud sounds (which receive neither amplification nor attenuation but are allowed to pass through unchanged).
By way of reference, the performance of the -720 patent circuit is similar to that of the compression circuits used in the compressors manufactured for the recording industry by the dbx corporation (Massachusets) under U.S. Pat. Nos. 3,789,143 issued Jan. 29, 1974 and 3,714,462 issued Jan. 30, 1973, both to David Blackmer. Although the performance is similar, the Blackmer circuits typically require .+-.12 Volts or more for proper operation, rendering them inoperable for hearing applications where operation on a 1.1 to 1.5 Volt cell is normally required.
The principal applicant and some of his colleagues have recently applied for four U.S. patents on circuit developments that permitted applicant's original -720 patent circuit, plus further improvements, to be realized in a silicon integrated-circuit version. These form the "four disclosures" referenced earlier.
Because the typical operation of WDRLC circuits in hearing aids for too-loud sounds, and because the WDRLC circuits typically spread the change in gain over a wide range of input signal levels (typically 50 to 60 dB), the principal applicant did not originally believe that adaptive compression would provide significant benefit to WDRLC hearing aid applications. Indeed, the Blackmer circuits used in commercial dbx compressors did not incorporate such variable-recovery-time circuits, nor was any such circuit taught in the Blackmer patents or in the Goldberg patent (U.S. Pat. No. 3,229,049 filed Aug. 4, 1960), the latter being the classic WDRLC hearing aid patent.
In extensive laboratory listening tests undertaken during development of the integrated circuit hearing aid amplifier described in the aforementioned four disclosures, a single recovery time was chosen as adequate for all listening conditions, which included pre-recorded traffic noise, cafeteria noise, cocktail party noise, and office noise, as well as quiet speech, and classical, jazz, and popular music. The teachings of the patent literature for WDRLC hearing aid circuits appeared confirmed: Variable-recovery-time circuits were needed only for output-limiting applications where high compression ratios are employed.
During several months of wearing the first production hearing aids incorporating the WDRLC integrated circuit described in the aforementioned four disclosures, however, the principal applicant noticed several occasions on which a "breathing" or "pumping" action of the compression circuit was audible. Other early wearers reported similar observations; one commented on the "busy" operation of the automatic gain control circuit. One situation in which the "pumping" became quite obvious was in a scientific lecture situation, where the hiss of the slide projector fan would "disappear" each time the lecturer spoke and then audible "reappear" quickly during each pause in his speaking.
In extensive further laboratory listening tests in cooperation with co-applicants Tedar, Hanke and Johnson at Telex Communications, only under the most carefully contrived listening situations was it possible to hear any improvement when the adaptive compression circuit of FIG. a was substituted for the single-recovery-time circuit of the -531 application. Nonetheless, applicant had his hearing aids wired so that the adaptive compression circuit of FIG. 1 could be selected by switch. The somewhat surprising result was that in actual "live" real-world listening situations, switching to adaptive compression made a marked improvement in the subjective quality of the compressor operation; an improvement that had been almost impossible to demonstrate in the laboratory situation. Moreover, switching to adaptive compression improved the ability of the hearing aid to accomodate short, intense transient sounds without distortion and without noticeably changing the gain of the hearing aid for the ongoing sounds of interest, much as disclosed by Hotvet for the application of adaptive compression to compression-limiting circuits.
As a result of the real-world listening tests, applicant's company Etymotic Research has negotiated a license with Telex corporation, owners of the Hotvet patent, under which Etymotic Research will incorporate the adaptive compression circuit into the WDRLC circuitry of its hearing aid amplifiers. The use of the adaptive compression circuit with WDRLC circuits appears to be novel, however, not having been anticipated in the Hotvet patent, and applicants respectfully request that a patent be granted on this improvement.